Biology 7.2.2 Epidemiology

Study set for EB Prebac

Disease definition

Disease definition

any condition which interferes with normal functioning of the body and impairs the health, opposite of health

Types of diseases

Congenital diseases (inborn and genetically inherited) and Aquired diseases (after birth and non-inheritable)

Congenital diseases two types and examples

Due to genetic mutation (e.g. haemophilia, sickle cell anemia), due to chromosomal or genomic mutations (Down’s Syndrome, Turner’s Syndrome)

Aquired diseases two types & many subtypes

Communicable or infectious diseases- air, water, food, physical contact; produced by bacteria, viruses, protists, fungi, parasites (e.g. ticks)
Non-communicable or non-infectious diseases- deficiency disease (diabetes), degenerative disease (arthritis), cancer, mental disease, allergy and autoimmune diseases, occupational diseases, due to enviornment (like UV, radiation)

WHO definition of health

Health is the state of complete physical, mental, social well-being and not merely the absence of disease, the ability to lead a socially and economically productive life

Biomedical concept of health

absence of disease, based on the “germ theory of disease”

Ecological concept of health

Dynamic equilibrium between human being and enviornment- disease is a maladjustment of the human organism to enviornment

Psychosocial concept of health

Also influenced by social, psychological, cultural, economic and political factors of the people concerned

Holistic concept of health

synthesis of all other concepts, recognizes strength of social, economic, political, and environmental influences on health; health as a unified/multi dimensional process involving the wellbeing of the whole person in context of his environment

Disease vs illness vs sickness

Disease- when something is wrong with bodily function, physiological/psychological dysfunction
Illness- presense of a specific disease, subjective state of the person who feels aware of not being well
Sickness- state of social disfunction, role that the individual assumes when ill

Different responsibilities for health

Individual- self care for maintaining own health,
Community- health care to the people by the people
State- constitutional rights
International- Health for All through PHC

Epidemiology definition

The study of factors affecting the health and illness of populations. Serves as the foundation and logic for interventions made in the interest of public health and preventative medicine

Epidemic definition

The occurence of more cases of a disease than expected in a given area or among a given group of people over a particular period of time

Pandemic definition

An epidemic occuring over a very wide area (usually multi country) and usually affecting a large proportion of the population

What is a mathematical model? (3 aspects)

Mathematical description of the real world, focuses on specific quantitative features of the scenario and ignores others (simplification), involved hypotheses that can be tested against real data

Why study epidemic models? (3 things)

To supplement statistical extrapolation, to learn more about the qualitative dynamics of a disease, to test hypotheses about e.g. prevention strategies, disease transmission, etc.

Why use SIR models?

new diseases emerge suddenly and spread quickly, effective an fast control measures are needed, allow you to predict when you don’t know

SIR meaning

Fractions of: susceptibles, infecteds and recovereds in a population

Simples SIR model

Susceptibles have no immunity, Infecteds have the disease and can spread it to others, Recovereds have recovered and are immune

β (beta) meaning in epidemiology

The infection rate- an infected individual comes into contact with βN other individuals per unit time (of which the fraction that are susceptible to contracting the disease is S/N)

γ (gamma) meaning in epidemiology

Removal/recovery rate- 1/γ is the mean period of time during which an infected individual can pass it on

Basic reproduction number definition

The average number of secondary cases cause by an infectious individual in a totally susceptible population, tells us how easy/difficult it is to eradicate an inection (easier to eradicate one that is low), R₀

Basic reproduction number formula

R₀ = infection rate/removal rate

What has to be the case for an epidemic to occur?

R₀>1

Individualistic definition for R₀

The number of infections an infected person would generate over the course of their infections if everyone they encountered were susceptible

Population definition for R₀

The average force for growth of infection in a population where everyone is susceptible

What is it when R₀<1

The infection cannot invade a population, infection control mechanisms are unnecessary, it will eventually die out by itself

What is it when R₀>1

(On average), the pathogen will invade that population, control measures necessary for prevent/delay an epidemic, every increasing the number of infected

What is it when R₀=1

Approximately the same number of individuals are infected with each new generation causing endemicity

R₀ for known disease

AIDS: 2-5, Smallpox: 3-5, Measles: 16-18, Malaria: >100

Factors affecting R₀ (3 things, their letters & ways to reduce them)

p- Transmission probability per exposure, can reduce by: screening blood, using gloves, condoms
c- number of contacts per time unit, can reduce by: isolation, sexual abstinence
d- duration of infectious period, can be reduced by medical interventions (e.g. TB)

Factors affecting R₀ formula

R₀ = p x c x d

Ways to enhance the SIR model (3 main ideas)

Consider additional populations of disease vectors (e.g. fleas, rats), consider an exposed but not yet infected class (SEIR model), can consider biased mixing, age differences, multiple types of transmission, geographic spread, etc.

What is sometimes used instead of an SIR model for sexually transmitted diseases, way to enhance it

SIRS, SIS, and double (gendered) models

Public health immunization program goal

Attain as close to 100% coverage as possible to prevent even one case from occuring

Herd immunity- what is it based on

The notion that if a population or group is mostly protected from a disease by immunization (>85%) then chance of a major epidemic occuring is limited

Herd immunity- what does it do, how does it work

Provides a barrier to direct transmission of infections through population, lack of susceptible people stops spread of a disease throughout the group

Reproduction number when you take into account vaccination and herd immunity, using V and S_0, what number does it have to be for an epidemic to occur

If only a fraction (S₀) is susceptible then the reproduction number is- R₀S₀ now this has to be >1 for an epidemic to occur,
If V = number of people vaccinated then- S₀=1-V
No epidemic can occur if: V>1 - 1/R₀

How to calculate R when a proportion (p) are immune (formula)

R = R₀ -(p x R₀)
That then needs to be >1 for an epidemic to happen

Example: if R is 2 then what does p need to be to prevent an epidemic?

P>0.5

European Center for Disease Prevention & Control (ECDC) brief description of function

Asseses and monitors emerging disease threats to coordinate responses

European Medicines Agency (EMA) brief description of function

Manages the scientific assessement of all EU medicines’ quality, safety and efficiency

Zoonosis definition

Any disease or infection that is naturally transmissible from vertebrate animals to humans